1. Field of the Invention
The present invention relates to an electrophotographic printing apparatus, and, more particularly, to an electrophotographic printing apparatus suitable for a printer using an electrophotographic process.
2. Description of the Prior Art
It has been known that, when it is intended for a conventional electrophotographic printing apparatus such a printer to change image density of an image to be printed, there are approaches to change the bias voltage for a developing roller in a developing section, or to vary amount of toner being used for development (first conventional example).
On the other hand, for example, Japanese Patent Application Laid-Open No. 62-47662 discloses an electrophotographic non-impact printer capable of two-color printing with the following arrangement (second conventional example). The printer employs an approach for varying density of characters or the like to be printed on a medium by controlling a driving current for a semiconductor laser through an operation panel provided external to the printer. This is because the semiconductor laser has a fixed relationship between its light intensity and image density. More specifically, this approach adjusts a variable resistor of the operation panel to vary the light intensity of semiconductor laser. The variation of light intensity causes variation of a potential on a photosensitive material, thereby finally changing the density of printing.
A third conventional example is disclosed in Japanese Patent Application Laid-Open No. 5-150602. It detects amount of bias voltage applied to a developing magnetic brush, intensity of light emitted from a light source and image density, normalizes them with a fuzzy inference processor, and controls the light source and the bias voltage of the printer based on that result.
As described for the first example, the conventional electrophotographic printing apparatus varies the bias voltage for the photosensitive material in order to vary the density of image. However, when the bias voltage is varied, there arises such a problem that so-called fog effect occurs on the printing medium, or that the printing medium tends to be blemished because the toner density is significantly varied.
In addition, the non-impact printer according to the second example has an operation panel therein for varying density of an image. Thus, if the printer is installed at a location remote from a computer, it is necessary for the operator to come to the site where the printer is installed every time he/she wants to change printing density, which is very inconvenient. Here, change of printing density means an action performed by changing the light intensity of semiconductor laser, rather than change of image density through image processing which is typically performed by a printer driver.
Furthermore, there arises a problem in a case where a computer is connected to a printer through a network, and, more particularly, where one printer is connected to a plurality of computers. If a user who operated the printer just before another user once changes light intensity of semiconductor laser through the operation panel, the light intensity of semiconductor laser is changed on a hardware level, so that the second user becomes impossible to perform printing under the previous setting. Thus, if the second operator performs printing as is without knowing the fact that the light intensity of semiconductor laser has been changed, he/she cannot obtain print quality under image density which he/she intends.